The properties of a granular polymer, i.e. polymer powder recovered from a polymerization reactor, substantially depend upon the properties of the catalysts used to prepare the polymer. In particular, the choice of the shape, size, size distribution, and other morphological properties of a solid catalyst used to prepare a granular polymer is important to the resultant granular polymer properties and to ensure process operability. This is particularly important in gas phase and slurry polymerization processes. Preferably a catalyst composition used to prepare granular polymers should be based on a procatalyst particle having good mechanical properties, including resistance to wear, abrasion and shattering during the polymerization process. Such good mechanical properties of the procatalyst particles assist in imparting good bulk density and uniformity to the resulting polymer product. Additionally, a preferred catalyst composition would generate one polymer particle per catalyst particle, replicating the particle size distribution of the procatalyst particles. Procatalyst compositions that produce such granular polymers with high catalyst efficiency, particularly for use in multiple reactor systems, would also be preferred.
Particulate procatalysts with good morphological properties, i.e. generally spherical, narrow to moderately narrow particle size distribution, and resistance to attrition, are desirable to provide good operation in a gas phase fluidized bed reactor. For examples, support materials which are subject to attrition, lead to the production of polymer fines. Polymer fines, however, are undesirable due to buildup in the polymerization equipment, problems with bed level control, and entrainment in the cycle gas, which may cause equipment failure, impaired operability, and reduced efficiency. High levels of fines can also cause problems in downstream handling of the polymer upon exiting the polymerization process, such as poor flow in purge bins, plug filters in bins, and safety issues. Such issues make elimination or reduction of polymer fines important to commercial operations, especially gas-phase polymerization processes. Moreover, the reduction of polymer fines is particularly important in a multiple series reactor system, where the composition of the polymers produced in the separate reactors is widely variable due to the importance of precise bed level control.
With respect to the preparation of polyethylene and other ethylene/α-olefin copolymers, it is preferred to produce polymer in separate reactors with both large molecular weight differences and relatively large differences in incorporated comonomer. To produce final polymers with the best physical properties, it is preferred to have one of the reactors produce a polymer with high molecular weight and incorporating a majority of any comonomer present. In the second reactor, a low molecular weight portion of the polymer is formed which may also have comonomer incorporated generally in an amount less than that incorporated in the high molecular weight portion. In some instances, the low molecular weight portion of the polymer is a homopolymer.
Depending on the order of production of the different polymers in the multiple reactor system (that is production of high molecular weight polymer first and lower molecular weight polymer second or vice versa), polymers produced from the fines arising from known catalysts will tend to have significantly different polymer properties than the bulk of the polymer granules. Without being bound to any particular theory, it is currently believed that the differing polymer properties are due to the fact that the fines also tend to be the youngest polymer particles in the reactor and hence do not achieve conformation to the desired polymer properties before transiting to the second reactor in series. Such a difference in the fine and bulk polymer properties leads to challenges in compounding the polymer into pellets for end use.
In particular, with known catalysts, the polymer fines are normally of significantly different molecular weight or branching composition compared to the bulk polymer. Although the polymer particles created by the bulk procatalyst and the procatalyst fines will melt at roughly the same temperature, mixing is nevertheless hampered unless the polymer particles have a similar isoviscous temperature. The polymer fines, which tend to have significantly different molecular weight and isoviscous temperature than those of the bulk polymer, are not readily homogeneously mixed with the bulk polymer. Rather, the bulk polymer and polymer fines form segregated regions in the resulting polymer pellet which may lead to gels or other defects in blown films or other extruded articles made from the polymer pellets.
Supports, such as silica gels, aluminas, silica aluminas and the like are generally good candidates within and/or on which to place the active catalytic species. However, the support material should also fracture into small enough pieces such that it is the support material is not visible in articles, such as films, produced using the polymer produced using the supported catalyst.
Known Ziegler-Natta type catalysts containing two or more transition metals and prepared from precipitated compositions may produce resins with broad molecular weight distributions which are useful to make films and blow molded articles. However, such catalysts result in low resin bulk density, poor polymer particle size and shape and require complicated preparation procedures.
Also known are silica impregnated multiple metal Ziegler-Natta type catalysts. One such known catalyst is prepared using a specific aerogel and has generally low catalyst activity in terms of residual Titanium content. Yet other known multiple metal catalysts result in low resin bulk density and also exhibit insufficient catalyst activity for operation in linked reactor systems.
Accordingly, there is a need for procatalyst composition possessing sufficient strength and solidity to resist fragmentation and fines generation in order to minimize polymer fines production in an olefin polymerization process, and particularly in a series reactor polymerization process. It is also desirable to produce polymers with improved properties, particularly broader molecular weight distributions that are suitable for blow molding and other processes for producing articles such as films, pipes and containers.